Thanks for all the advice. One more question...do I need to line it with another fabric?
Welcome to the Quilting Board!

Thanks for all the advice. One more question...do I need to line it with another fabric?
No, You don't need to line it. You might want to line it depending on how you prefer your robes to feel. Lining it with satin will give it a different texural feel to your body and make it warmer. You could make it reversible if you wanted. It's your robe; what do you want the robe to be?
I have some chenille just sitting around. I was going to make teddy bears with it. I did wash it and it was very linty. A serger sounds like a good idea if you have one. Also the suggestion to avoid set in sleeves is a good one.
Sue